DozyWannabe
It went all the way back in this case because the elevators were no longer sufficient to maintain the requested attitude as the speed bled off. The THS had moved approximately 5 degrees nose-up at the apogee of the climb. From that point onwards, the stick is held halfway back by the PF for a further 15 seconds, released for about 4 seconds with a small "blip" forward and then slammed against the back stop for an eye-watering 34 seconds. In short, the THS went where it was told to go by the PF. If he'd released the stick prior to the apogee, it would likely have stayed at a max of 5 degrees NU.
You still have not fully understood the autotrim, THS, NzLaw without protections. Unfortunately BEA has not added normal acceleration in the graphs of the final report, therefore we have to deal with the bad graphs from Interim report 3.
From 02:10:25 until 02:10:50 the normal acceleration was below 1g. When PF after the stall warning 2 selected TOGA, the normal acceleration increased for a short time of about 10 seconds above 1 g. From that point on the normal acceleration was uninterrupted below 1 g until 02:12:00. When PF applied SS full NU at 02:11:40, the THS had already reached near full NU limit and the elevators moved now full NU. Despite those NU commands and despite the position of the THS the normal acceleration stayed below 1 g for another 20 seconds, until 02:12:00.
During all those times, where the aircraft was not able to maintain even 1 g, (and that started already before the apogee of the climb) the trim and the elevators would have moved on their own to achieve this 1 g with hands off SS neutral= 1g.
To stop the trim from further traveling, the SS would have to be moved and held at the present normal aceleration (which was below 1 g). To get the trim moving ND, the SS would have to be held to demand a g value well below the present normal aceleration.
Interesting, after PF started putting the SS to the NU stops and reduced the power at the same time to idle, the pitch of the aircraft decreased from +15° to -10° within 10 seconds. Power change and maybe the elevator deflection to full NU changed the forces of the airframe. What that did to the mental picture of the PF would be interesting to know. If PF would have gone to full ND at that point, the he might have had a chance to initiate recovery.
After the aircraft dropped outside the flightenvelope, which happened after the stall warning 2, any command except a definite ND command was bad, but the amount and the duration of the NU input or even a SS neutral input would not have saved the day and did not agrevate the NU travel of the trim. and the elevators. Because those were already maxed out by trying to maintain 1g.
BEA FR 2.1.3.2 Exit from the flight envelope
Subsequently, the position of the sidestick, maintained in its nose-up or neutral position, continued to exacerbate the situation and made the recovery uncertain, even impossible.
BEA FR 2.1.3.5 End of the flight
At about 2 h 12, descending though FL 315, the aeroplane’s angle of attack was established around an average value of about 40 degrees. Only an extremely purposeful crew with a good comprehension of the situation could have carried out a manoeuvre that would have made it possible to perhaps recover control of the aeroplane. In fact, the crew had almost completely lost control of the situation.
Up until the end of the flight, no valid angle of attack value was less than 35°.